1. Field of the Invention
The present invention relates to a circuit breaker capable of performing a breaking operation for line protection or for opening and closing a load, or when an overload and a short circuit have occurred, and particularly, to a circuit breaker having a shaft unit, the circuit breaker capable of minimizing damages due to an accident current by rapidly performing a tripping operation when an accident current has occurred.
2. Background of the Invention
Generally, a circuit breaker normally maintains a conductive status when a rated current flows, but performs a breaking operation when an accident current occurs. More concretely, a fixed contact and a moveable contact maintain a closed status when a normal current flows. However, when an accident current has occurred, the fixed contact and the moveable contact are separated from each other to break the accident current as a trip unit detects the accident current and transmits a signal to a switching mechanism. Then, the switching mechanism is released.
Here, it takes several minutes for the switching mechanism to be released. The shorter the time, the more a breaking function of the circuit breaker is maximized. While the accident current is applied to the fixed contact and the moveable contact, an electromagnetic repulsive force is generated between the fixed contact and the moveable contact. As a result, the fixed contact and the moveable contact are separated from each other.
Once the moveable contact has been separated from the fixed contact by the electromagnetic repulsive force, the trip unit detects the accident current to limit the accident current before the switching mechanism is released. This may reduce the amount of energy applied to the circuit breaker to prevent damages of the circuit breaker, thereby maximizing a breaking function of the circuit breaker.
FIG. 1 is a sectional view of a circuit breaker having a shaft assembly in accordance with the conventional art, and FIG. 2 is a disassembled perspective view of a latch unit and a moveable plate unit of the shaft assembly in FIG. 1.
As shown, the conventional circuit breaker comprises a trip unit 1 configured to detect an accident current, a switching mechanism 2 configured to separate a moveable contact 21a from a fixed contact 5a by receiving a signal indicating the occurrence of an accident current from the trip unit 1, and a shaft assembly 3 configured to perform a closing or tripping operation by the switching mechanism 2.
The shaft assembly 3 includes a shaft 10, a moveable plate unit 20 having the moveable contact 21a detachably mounted to the fixed contact 5a fixed to the circuit breaker, and rotatably coupled to the shaft 10, and a latch unit 30 disposed between the shaft 10 and the moveable plate unit 20 so that the moveable plate unit can maintain a closed or tripped status.
The moveable plate unit 20 consists of a moveable plate 21 having the moveable contact 21a welded at one end thereof, and having a pin forcibly inserted at another end thereof, a connecting plate 22 coupled to the shaft 10 and having the moveable plate 21 rotatably coupled thereto, a connecting pin 23 configured to couple the moveable plate 21 and the connecting plate 22 to each other, connecting springs 24 configured to generate a contact pressure by pushing the moveable plate 21 and the connecting plate 22 from two sides, and washers 25 configured to prevent separation of the connecting springs 24.
The latch unit 30 consists of a limit latch 31 supported by the shaft 10 and the moveable plate 21, a latch spring 32 implemented as a compression spring so as to elastically support the limit latch 31, and a limit pin 33 penetratingly formed at the shaft 10 and the limit latch 31 and configured to rotatably support the limit latch 31 by the shaft 10.
Unexplained reference numeral 5 denotes a fixed plate, 11 denotes a pin groove, and 21b denotes a moveable plate pin.
The operation of the conventional shaft assembly will be explained.
As shown in FIG. 3, a compression force of the limit spring 32 assembled to the shaft 10 is transmitted to a moveable plate pin 21b forcibly inserted into the moveable plate 21 through the limit latch 31. As a result, the moveable plate 21 maintains an OFF status.
As shown in FIG. 4, when the circuit breaker is in a closed (ON) status by the switching mechanism 2, the shaft assembly 3 is rotated to allow the moveable contact 21a of the moveable plate 21 to come in contact with the fixed contact 5a of the fixed plate 5. Here, the compression force of the limit spring 32 is transmitted to the moveable plate pin 21b forcibly inserted into the moveable plate 21 through the limit latch 31. As a result, a contact pressure is generated between the fixed contact 5a and the moveable contact 21a. 
As shown in FIG. 5, in the occurrence of an accident current, a contact repulsive force is generated between the fixed contact 5a of the fixed plate 5 and the moveable contact 21a of the moveable plate 21. As a result, the moveable plate 21 is lifted up. Here, the moveable plate pin 21b moves along an operation surface (outer surface) of the limit latch 31, which is located on positions different from the operation surface of the limit latch 31 shown in FIGS. 3 and 4. Then, the compression force of the limit spring 32 assembled to the shaft 10 is transmitted to the moveable plate pin 21b forcibly inserted into the moveable plate 21 through the limit latch 31. As a result, the moveable plate 21 is lifted up (current limiting status).
However, the conventional shaft assembly consecutively operated as shown in FIGS. 3 to 5 may have the following problems.
Referring to FIG. 6, a contact pressure generated between the fixed contact 5a of the fixed plate 5 and the moveable contact 21a of the moveable plate 21 will be explained. It takes about 17 seconds for the moveable contact 21a to be lifted up from a contact time point between the fixed contact 5a and the moveable contact 21a. A contact pressure when the circuit breaker in an ‘ON’ state is in the range of 2.5˜2.7. More concretely, it takes a long time from the contact time point between the fixed contact 5a and the moveable contact 21a to the time point when the moveable contact 21 is lifted up. This may cause a time duration for which an accident current flows to be long. Accordingly, the amount of energy applied to the circuit breaker is increased to degrade a breaking function of the circuit breaker. Furthermore, reliability on a conductive state between the fixed contact 5a and the moveable contact 21a may be lowered due to a large change of the contact pressure.